Not applicable
1. Field of the Invention
This invention relates to a bearing system that accommodates angular movements occurring during the operation of a linear actuator, and more particularly to such a bearing system utilizing annular, non-spherical bearing surfaces, allowing motion about three mutually perpendicular axes during the operation of a linear actuator in a lifting structure for a ladle that contains liquid steel in a continuous casting installation.
2. Background of Invention
In the continuous casting of steel, molten steel is transferred to a continuous caster by ladles weighing typically from 200 to 400 tons, which includes the molten steel. The temperature of the molten steel is critical to the casting process, and so it is imperative that a ladle is tapped for the casting process according to a schedule established to prevent the cooling of the molten steel below a desired casting temperature and to avoid the need to return the molten steel to the steel making furnace for reprocessing and the consequential stoppage of the casting process. An unplanned stoppage of the casting process not only results in a consequential production loss but also in additional operating costs to restart the continuous casting process. The complex process of continuous casting begins at the caster, where molten steel flows from a ladle into a tundish, and then into one or more continuous casting molds. Maintaining a steady flow of molten steel into the continuous casting mold is essential, so as not to disturb the delicate balance of cooling, containment of a liquid steel core in the newly formed solid shell, and casting speed required for proper solidification. The volume of liquid in the tundish is selected to maintain an operating level unaffected by minor fluctuations in the liquid flows formed by the floating out of impurities of slag and the flow of molten steel into the mold. Another factor affecting the volume of liquid steel contained in the tundish is the need to accommodate a momentary interruption of the flow of molten steel from one ladle to allow the sequencing of ladles to the caster and reestablish a flow of molten steel from a second ladle.
A typical sequencing of ladles is started by first increasing the flow of molten steel from a first ladle slightly before the ladle is empty to raise the liquid steel level in the tundish slightly above an operating level. When the flow from the ladle changes from molten steel to slag, the slag flow is detected and a slidegate is immediately closed to stop the flow of slag. A pouring tube is disconnected from the ladle nozzle and the ladle is moved away from the casting position. At the same time a second ladle is brought into cast position and a pouring tube is connected to the ladle nozzle of the second ladle whereupon opening of the slidegate initiates the flow of steel into the tundish. The entire sequence, from the stoppage of the flow of molten steel in one ladle to the establishment of a flow of molten steel in a replacement ladle, must be completed before the liquid level in the tundish has been depleted to a certain critical level, below which the quality of the cast steel strand is adversely affected. The sequence of changing the supply of molten steel from one ladle to another is normally accomplished within a very safe time margin. There are instances where reliance on the safe time margin is necessary such as, for example, when the ladle nozzle of the second ladle fails to open due to the freezing of steel in the pouring channel of the ladle exit port. It is a customary practice to use an oxygen lance, which is consumed in the process to remelt the solidified steel in the ladle exit port and start the flow of steel. The pouring channel is sometimes blocked by a large column of solidified steel, in which event it is necessary to raise the ladle to a greater elevation above the tundish to allow greater access to the ladle exit port for the continuous feeding of an elongated oxygen lance which is consumed while remelting the column of solidified steel.
To allow more time for the variable time-consuming use of the oxygen lance, designers have tried to decrease the time needed for other functions such handling of the pouring tube and, in particular, for movement of the ladles. Two typical devices employed in the efficient exchange of ladles are a ladle car and a ladle turret, both of which may employ an actuator for lifting a ladle. A ladle used to supply steel to the tundish of a continuous caster normally has two separate structures for supporting the ladle. The first structure is a pair of trunnions extending from the ladle along a horizontal axis. They are located above the center of gravity of the ladle and are engaged by the J-hooks of a hot-metal crane to transport the ladle to the caster. The second structure is a pair of elongated horizontal seats located below the trunnions. The seats are used for supporting the ladle on a ladle car, ladle turret or stationary stand. The transfer of a ladle from the hot metal crane to a ladle car, ladle turret or stationary stand normally occurs with impact loading on the elements of the support system in the ladle car, ladle turret or stationary stand. A lifting mechanism on the ladle car or turret normally includes means to stabilize the ladle supports and thus also the ladle against tilting during the lifting travel. In addition to sustaining the vertical loading, the lifting mechanism also receives lateral loading due to incidents of positing and negative accelerations due to the movements of the ladle. The vertical and lateral loads on the lifting mechanism produce critical structural deflections that must be considered in the design of the bearing elements of the lifting mechanism.
Electro-mechanical lifting of a ladle on a ladle car, ladle turret or stationary stand has been widely used in the past and provides that each of the two ladle seats is supported on a lifting bridge. The lifting bridges are supported at each end on screw jacks. The four screw jacks are synchronized and driven by universal spindle shafts and gearboxes by one electric motor. The electromechanical lifting system is very reliable for a short lifting distance at a slow speed, but to move a ladle over an extended lifting distance at a faster speed requires heavy-duty ball screw jacks, which are not commercially available. The special nature of the ball screw jacks is very costly in terms of both initial equipment and maintenance. The use of hydraulically powered actuators for lifting a ladle has been employed as a direct replacement of the electromechanical system. The overall structure of the mechanism is very similar and essentially involves replacing each screw jack and associated drive components with a hydraulic cylinder. Synchronization of the four hydraulic cylinders when controlled within the hydraulic system is susceptible to wear of system components and leakage of hydraulic fluid. An improved control system includes the use of an electronic feedback and position controls, but integration of the control system is costly and requires adherence to a strict maintenance schedule.
A third method of lifting a ladle on a car or turret is a combination of hydraulic actuation with mechanical synchronization. This method is chosen for the purpose of illustrating and describing the present invention hereinafter. The mechanical synchronization is accomplished by the use of two parallelogram linkages, one positioned in each of two vertical and parallel planes containing support points for the elongated horizontal seats of the ladle. A rigid frame is used to tie the upper links of the two parallelogram linkages together so that a single hydraulic cylinder positioned near the center of the rigid frame provides the lifting force. One end of each upper link is supported at a stationary point by a bearing on each side of the vertical support structure of the ladle car or turret. The lower link of each parallelogram linkage serves to stabilize and maintain the ladle supports, keeping them horizontal throughout the lifting movement. Hydraulic actuation provides the desired speed without the maintenance problems of electromechanical drives. The single hydraulic actuator is unaffected by wear in the hydraulic system and allows less stringent hydraulic maintenance requirements than in a system with hydraulic synchronization.
The lower end of the hydraulic lifting cylinder is mounted by a bearing on the stationary frame used also to support the lifting linkage. The upper end of the cylinder is attached to the upper link frame by a second bearing. As the cylinder extends upward, the upper link frame rotates counter-clockwise about the bearing on the stationary frame. While the cylinder remains mostly vertical, the rotation of the upper link frame causes a slight pivotal motion of the cylinder about the lower cylinder bearing. The upper cylinder bearing experiences significant rotation approximately equal to that of the upper frame. Attached to the extended ends of the upper links, which lie at opposite sides of the cylinder, are ladle support arms. The arms rise as the upper link frame rotates. The lower links stabilize the ladle support arms, keeping the ladle support surfaces always horizontal as they rise. In a theoretical sense, all of the motion described above occurs in parallel planes, each containing one of the parallelogram linkages. However, due to structural deflections and manufacturing tolerances, there are small components of movement into and out of these theoretical planes, which must be accounted for in the design of the bearings used in this parallelogram linkage. The use of spherical bearings is known in the art to allow rotation about the pivot shaft of the bearing and slight pivotal movement about the axes perpendicular to the shaft.
A spherical bearing is suitable at all pivot positions in the parallelogram linkage except for the bearing needed to join the upper and lower ends of the actuator cylinder to the ladle car or turret and the upper link frame. The bearings for the actuator cylinder are plagued with problems of design, material selection, manufacture, installation, and maintenance. In FIGS. 1-3 there are illustrated typical spherical bearings 1 and 2 at the rod and cylinder ends, respectively, of a piston and cylinder assembly 3. Each bearing 1 and 2 comprises an inner ring 4 and an outer ring 5 formed with mating spherical surfaces used for the transmission of all forces provided by the piston and cylinder assembly 3. The inner ring 4 is free to rotate in all directions about the mating spherical surfaces relative to the outer ring 5. Likewise, if the inner ring is held stationary, the outer ring is free to rotate in a similar fashion. The inner ring of the bearing 1 is mounted in a fixed relation on a pivot a shaft 6, and the outer ring 5 is mounted in a fixed relation in a housing structure 7 on the free end of the piston. The inner ring of the bearing 2 is mounted in a fixed relation on a pivot a shaft 6 and the outer ring 5 is mounted in a fixed relation in housing structure 8 used to secure the cylinder to a support. In their nominal positions, the bores of the housing structures 7 and 8 and the axes of the pivot shafts 6 are parallel and the housing structures may rotate freely about the axes of the shafts and vice-versa. Since the rotating surface is spherical, the axes of the housing bores and the shafts may skew, twist to some degree, which is limited by the proportions of the bearings, the housing structures, and the shafts. The skewing is the result of structural deflections and misalignments between the housing structures and shaft axes, and is normally a very small magnitude as compared with the magnitude of the rotation about the axes. It is relevant to the present invention that all of the motion described above occurs on the same surfaces, namely the spherical surfaces.
Material combinations typically employed in the spherical bearing inner and outer races are steel-on-steel and polytetrafluoroethylene on steel. Since the lifting load is unidirectional, steel-on-steel is difficult to lubricate, and so the self-lubricating polytetrafluoroethylene material is preferred. The polytetrafluoroethylene material performs poorly under impact loading and must be protected from contaminants, including moisture. A typical moisture barrier is an external grease-filled cavity separated from the polytetrafluoroethylene material by a seal identified by reference numeral 9. Grease lubricants and moisture are harmful to the polytetrafluoroethylene material and thus the performance of the seal is very important. However, the use of grease in the seal is necessary to provide a moisture barrier. While the steel-on-steel can withstand extreme impact loads, its poor lubrication results in a low dynamic capacity. With these considerations, for either material a larger bearing must be chosen to suit all requirements. Proper functioning of spherical bearings requires a precise mating of the spherical surfaces between the inner and outer rings 4 and 5. Costly tooling and processes are required to form the spherical surfaces. The spherical bearing is typically installed with an interference fit on the shaft to prevent rotation of the inner ring on the shaft. This requires precise alignment of mating parts and a heavy pressing force on the shaft to avoid consequential damage. Steel-on-steel spherical bearings are susceptible to galling in the presence of even the minutest metallic particles, which may be the product of normal wear and must be completely purged from the bearing by frequent lubrication.
In the application of a typical ladle-lifting cylinder, it is desirable to minimize the overall height of the cylinder assembly. The height of the cylinder affects the surrounding structural frames of the support and lifting systems, which become quite irregular as the height increases. Two areas with opportunities for height reduction are minimizing the outside envelope of the upper and lower bearing assemblies of the lift cylinder.
The objectives of the invention are:
1. To provide a bearing system that allows reliable lifting of ladles with extended maintenance intervals;
2. To allow replacement of individual bearing elements without an extended interruption to the casting process;
3. To provide a bearing system that allows indefinite reconditioning and reuse of structural elements such as shafts;
4. To provide a bearing system that reduces the space requirements for the lift actuator assembly;
5. To simplify the bearing system of the ladle lift mechanism;
6. To relax the manufacturing tolerances of the bearing system and to eliminate complex processes for assembly;
7. To allow manufacture of bearing components with conventional machinery operations such as turning, boring and milling;
8. To separate the functions of the bearing into discrete axes with different designs suited to independent requirements;
9. To allow the use of conventional engineering materials and processes to produce bearing elements;
10. To eliminate the need for complex sealing arrangements and strict protection systems for the bearing elements;
11. To provide a bearing system that reduces the strains on the lift system during sequencing of ladles;
12. To improve stress conditions within the bearing assembly, allowing smaller components and better lubrication;
13. To provide a bearing system that reduces overall cost of the lift system equipment;
14. To provide a bearing system that reduces maintenance costs for the lift system; and
15. To reduce the cost of individual replaceable parts of the bearing elements.
According to the present invention there is provided the combination of an actuator including an actuator shaft having annular bearing surfaces centered about a first axis for free rotation about the first axis during controlled excursions of translating motion by the actuator shaft along the first axis, and a bearing system comprising: an attachment shaft joined by spaced apart cylindrical surfaces lying at opposite sides of the first axis to a clevis for only sliding rotational motion about a second axis centered along the cylindrical surfaces; and an anchored pivot block with an elongated annular load-bearing surface intercepted by the first axis and joined in a force transmitting relation with the actuator to allow only rolling rotational motion without sliding from a third axis extending along the annular load-bearing surface.
According to a further aspect of the present invention there is also provided the combination of an actuator support base, an actuator including a piston having an annular bearing slidable in an elongated cylinder along a first axis, an actuator shaft extending from the piston along an annular bearing secured to the cylinder for free rotation of the piston and actuator shaft about the first axis during controlled excursions of translating motion by the piston along the cylinder, and a bearing system comprising a first attachment shaft joined by spaced apart cylindrical bearings to spaced apart arms of a first clevis for only sliding rotational motion about a second axis centered along the cylindrical bearings, a second attachment shaft secured to spaced apart arms of a second clevis, a cylindrical bearing mounted on the second attachment shaft between the spaced apart arms of the second clevis for only sliding rotational motion about a third axis centered along the cylindrical bearing and parallel with the second axis, a first pivot block anchored for support by the actuator shaft and having an elongated annular load-bearing surface defining a fourth axis extending along a force transmitting junction with the first attachment shaft, and a second pivot block anchored for support by the actuator support base and having an elongated annular load-bearing surface defining a fifth axis extending along a force transmitting junction with the cylindrical bearing of the second attachment shaft, the fourth axis and the fifth axis being parallel and the elongated annular load-bearing surface of each of the first pivot block and the second pivot block allowing only simultaneous rolling rotational motion without sliding from each of the fourth axis and the fifth axis.